what
causes menstruation

Okay, so I know that most people reading this have seen the movies
and the film strips in junior high and have taken the mandatory biology
courses, of which human reproduction is surely explained again, but
consider this a refresher course. Lets start at the beginning:

When does it start:

Not every girl begins menstruating at the same age. Women, think back
to your own experiences. Remember your friend Jennifer who "started"
at age nine, you at 13 and Alice, lucky gal, who waited until you all
were 16.

Why such a range of ages? The experts are not quite sure. At the average
age of twelve, the hypothalamus begins releasing hormones that trigger
the ovaries to get into gear and produce an egg. What causes the release
is the question. One theory is that fat content is what triggers it
all. Natalie Angier puts forth the "rule of thumb... that when
a girl reaches approximately one hundred pounds, she pubesces, regardless
of height or even her age" (178-179).

Day one of the menstrual/ovulatory cycle is usually counted as the
first day of menstruation, or the shedding of the uterine lining (Bio,
926). For the ovaries it is a quiet time, where relatively no hormones
are released (Angier, 180).

Now begins the beginning of ovulation and what is known as "the
period". The menstrual flow occurs for an average of five days
during this time (Bio, 926). The lack of hormonal output from the ovaries
signal hormonal output from the pituitary gland. It signals the follicles
in the ovaries where eggs are "born" to get started. About
twenty of them begin, but by day ten, on average, only one of them remains.
The body allows only one follicle's egg to continue maturing, usually.
The natural occurrence of multiple births is the result of more than
one egg maturing past this stage (Angier 180-181).

Now begins the end of ovulation. The "liberation of the egg"
occurs on a hormonal release from the pituitary gland. The follicle
erupts and the egg is released into the waiting fallopian tube, where
it begins its journey to the well-lined uterus. According to Angier,
this can happen "on day twelve or fourteen or thereabouts"
(181). It also marks the end of the proliferative phase, which is begins
at the end of menstrual flow. This is the stage in which the endometrium,
or lining in the uterus, grows thick as its arteries enlarge to provide
nourishment for possible fertilized egg (Bio, 926).

Now, as the egg travels through the fallopian tube, waiting to be fertilized,
the follicle it erupted from begins to undergo a change. It develops
the corpus luteum, or "yellow body". The corpus luteum is
responsible for furthering the thickening of the endometrium and preparing
the rest of the body for a possible pregnancy. If conception does not
occur, the corpus luteum disintegrates and signals the endometrium.
If pregnancy does occur, the presence of the corpus luteum keeps the
cycle from starting again (Angier, 182-183). This is the secretory phase
of menstruation, where the endometrium continues to grow. It lasts approximately
two weeks and ends with the shedding of the top layers of the endometrium,
or menstrual flow: back to day one (Bio, 926).

The menstrual cycle ends at a time known as menopause. The relief,
or horror, of "The Change." It occurs, on average, between
the ages of forty-six and fifty-four. It is started by the ovaries lack
of responsiveness to the hormonal signals produced by the pituitary
gland (Bio, 927).

What effects changes in the norm:

While the menstrual cycle cannot be controlled by any form of mind
over matter, there are events that could occur to stop or delay its
timing. The most variable of times in the cycle is that of ovulation.
It can be delayed due to physical stress, such as malnutrition or the
flu. Angier puts this phenomenon into logical terms, "if you're
seriously ill, you need to focus on getting well. You can't afford to
divert energy to a pregnancy" (183-184).

One very intriguing, and widely debated idea, is that of menstrual
synchrony, or the idea that women in close contact with one another
actually have a shift in their cycles so that the two occur simultaneously.
The original study published on this occurrence first came out in the
journal Nature in 1971. It claimed that menstrual synchrony did
occur. According to Angier, later studies confirm the first, some refute
the results and one even proves the opposite (186). As you can see,
it is still a bit of an unknown arena.

One study, though, goes a bit further than simply trying to observe
the effect: it attempts to find the cause. Martha McClintock, author
of the first study, published results in Nature in 1998 from
an experiment her team undertook. Angier summarizes the results nicely:

'The scientists showed that if they took swabs from the armpits of
women at different points in their ovulatory cycle and applied the
swabs to the upper lips of other women, the donor secretions could
act as pheromones, as odorless chemical signals. The secretions either
hastened or prolonged the cycles of many, though not all, of the women
exposed to them (188).'

Angier claims that the though not all women were affected, enough were
to prove statistical significance. She also claims that this result,
though not completely understood, would account for the widely varying
results of other menstrual synchrony studies (189).

Of course, women taking any form of the birth control pill in the prescribed
manner, are regulating themselves. They are ingesting the hormones that
their bodies would be producing in order to "fool" themselves
out of actual pregnancies. Any variation in their administration of
the pill would cause a change in their cycle, but if no variations occur,
the are "unnaturally" regular (Planned Parenthood Pamphlet).